The present invention relates generally to the design and construction of a gear reduction gearbox for a gas turbine engine. More particularly, the present invention has one embodiment wherein the gear reduction gearbox utilizes an epicyclic face gear drive unit connected to a gas turbine engine to reduce shaft speed and increase the output torque. Although the present invention was developed for use in a gas turbine engine, certain applications may be outside of this field.
Gas turbine engine designers generally integrate a gear reduction gearbox with the engine in order to reduce the output shaft speed, and increase the torque delivered to an output device, such as a fan. It is well known that gas turbine engines are high speed rotary equipment having components including an output shaft revolving at speeds from about 5,000 to 50,000 revolutions per minute. Sometimes, in order to harness the power from the output shaft of the gas turbine engine a main gear reduction gearbox is coupled to the engine to decrease shaft rotation speed and increase output torque. Main gear reduction gearboxes include gear sets therein for reducing the shaft speed during the transmission of power from the gas turbine engine to the output devices, such as a fan. The transmission of power from the gas turbine engine to the output device imparts substantial loads on the bearings and gears of the gearbox.
Historically, designers of main gear reduction gearboxes for gas turbine engines have relied upon multi-stage reduction gearboxes not having bevel gear sets. A limitation associated with bevel gear sets is the inability to efficiently share the load transmitted through the gearbox. Absent load sharing within the gearbox there is an increase in stress on the individual gear and the bearing associated with the gear, therefore often requiring an increase in the bearing and gear size. The technological difficulties associated with load sharing in bevel gear drives stems directly from having to precisely align the mating gears of the set, and therefore the use of multiple pinion gears on a common shaft can create conflicts in alignment. Alignment conflicts serve to increase the complexity of manufacturing and servicing the gearbox.
The application of gas turbine engines as a propulsion means for an aircraft often creates design parameter conflicts, such as the need for a durable long life gear train and the necessity to minimize the volume and weight of the respective engine. Prior designers of gas turbine engine gear reduction gearboxes have generally used multi-stage gearboxes to effectuate significant shaft speed reduction. One common limitation associated with bevel gear sets is that their gear reduction ratio is below 8:1 due to manufacturing constraints. Therefore in order to obtain a higher gear reduction ratio involves adding additional stages that add weight, unit manufacturing costs, and often complicates construction and causes alignment problems between gear sets.
Apart from kinematic limitations associated with bevel gear sets there are additional constraints that designers of gear reduction gearboxes must address. One additional constraint is that bevel gear sets require precision alignment in the axial and radial direction. The alignment precision between the pinion and meshing gear often creates assembly difficulties, thereby increasing the cost to manufacture and service the gearbox. Further, the precision alignment parameter often limits the designers flexibility in gear set arrangements.
In many gas turbine engine gear reduction gearboxes the individual gears are not stress limited, but in reality the bearings are the limiting factor due to fatigue failure. Thus, in many prior gear reduction gearbox designs the bearing fatigue life dictates increasing the size of the bearing, which thereby increased the gear size. Further, many prior gearbox designs must fit within a prespecified volume and the utilization of bevel gear arrangements often precludes using larger bearings because of their impact on volume, thereby limiting the service life of the components.
Even with the variety of prior gas turbine gear reduction gearboxes there remains a need for an improved gear reduction gearbox. The present invention satisfies this need in a novel and unobvious way.